1. Field of the Invention
The present invention relates generally to velocimeters, and more particularly to an electrode array electromagnetic velocimeter that measures the velocity distribution characteristics of a liquid as it flows over a solid surface. The invention can be used as a general purpose velocimeter in conjunction with a conductive liquid.
2. Description of the Prior Art
There are many situations that arise in military, industrial, and commercial enterprises in which it is important to be able to measure the velocity of a liquid. Over the years, a variety of methodologies have been utilized to solve this problem. For instance, one of the first accurate methods, called the "salt velocity" method, involves introducing a quantity of salt into the flow and detecting its movement past two spaced points in the flow using electrical sensors. However, this method is limited to controlled situations.
Another common method for measuring fluid velocity uses a pressure probe known as a "pitot tube" in which the static and total pressures at a point are measured. The fluid velocity is calculated from Bernoulli's equation or from a calibration curve, using the difference between the total and the static pressures, which difference represents the kinetic or velocity energy. The drawbacks associated with the use of such a pressure probe are that the probe is intrusive on the velocity field itself, is dependent on fluid properties, and provides only an indirect measurement of velocity.
Another form of velocimeter involves the use of a hot wire or film, but such a device can also effect the velocity by its very presence. It provides a very indirect measurement of velocity, since heat transfer from the hot wire is measured and the velocity is calculated by means of an experimental correlation, which correlation is dependent on fluid property variables.
The laser doppler velocimeter provides yet another means of fluid velocity measurement by use of the frequency shift of reflected laser light from solid particles in the flow field. However, because of the size of the apparatus, it is usually limited to a laboratory situations. Another constraint imposed by the use of the laser system is that the fluid must be transparent to the laser light and that the solid particles must be small enough to accurately follow fluid streamlines.
It has been recognized for some time that a non-obtrusive velocimeter can be constructed using electromagnetic (EM) principles if the fluid is an appropriate electrical conductor. The velocity field of such a fluid can be found by imposing a suitable magnetic fluid on the flow and then measuring the resulting induced voltage between two electrodes placed in the fluid or on a solid insulated surface in contact with the fluid. Such an electromagnetic velocimeter provides a direct measure of the velocity field. It is "direct" in the sense that a basic law (Faraday's Law) is used to measure the velocity field, rather than relying on empirical correlations. This velocity field measurement is completely independent of the usual fluid properties (e.g., viscosity). The magnetic field will not distort the velocity field, so long as the magnetic Reynolds number is kept small. This would be the case with seawater, for instance, but not with a liquid metal, such as mercury.
One difficulty in using an electromagnetic velocimeter is in obtaining an explicit expression for velocity at a point. This can be seen from the so-called "flow meter" equation, known as the Shercliff-Bevir equation, which was given by J. Bevir and published in "Journal of Fluid Mechanics" Volume 43, 1970, at page 577. This equation gives velocity explicitly as: ##EQU1## where .DELTA..phi. is the induced voltage difference between two electrodes, v is the volume of integration in the fluid, V is the fluid velocity vector in terms of dv, and W is the weight vector (the fluid meter geometry factor) given by EQU W=B.times.J.sub.v ( 2)
where B is the magnetic field flux density vector and J.sub.v is the virtual current vector as defined by Bevir. The induced voltage .DELTA..phi. in Equation (1) is the output voltage signal of the electromagnetic velocimeter and is implicitly related to the fluid velocity field through the integral in Equation (1).
In U.S. Pat. No. 4,484,146 issued to Bruno et al., an electromagnetic velocimeter was presented that takes into account the matter of induced currents and end effects. However, this patent has only two pair of parallel line electrodes, each mounted orthogonal to the other. Thus, if fluid motion is in a direction normal to one pair, the other pair (parallel to the fluid motion) will provide the only voltage output .DELTA..phi. for the velocimeter. By Faraday's Law, the normal pair will give no signal. Thus, from Equation (1), it can be seen that it would be difficult to obtain detail explicit information about the fluid velocity, based on only a single value of .DELTA..phi.. Although several of these velocimeters disclosed by Bruno et al. can be used at the same time, each would have a different volume over which the velocity vector V in Equation (1) would be measured.